Epoxies are an important class of thermosetting polymers. They have diverse applications including adhesives, structural materials, paints coatings, potting, printed circuit boards, microelectronic encapsulation, the aerospace industry, and other consumer goods. Epoxy resins are hardened or cured by a cross-linking reaction using one of three methods. The chemistry of epoxy curing is explained in great detail in the Handbook of Composites (edited by S.T. Peters, Chapter 3, pp 48-74, published by Chapman & Hall, 1998). The properties and applications of cured resin are greatly influenced by the choice of the hardener formulation or the method of curing.
One method is simply the reaction of the epoxy resin with itself (i.e. homopolymerization) via a ring-opening polymerization mechanism of the epoxy groups. The self-curing of epoxy resins usually requires an elevated temperature but can be initiated with either a Lewis acid or a Lewis base catalyst (as opposed to a curing agent).
In the second method, the epoxy resin is cured with a cyclic acid anhydride. The anhydride can react with the epoxy group, pendant hydroxyls, or residual water to form a carboxylate intermediate, which then reacts with the epoxy group, causing a self-perpetuating reaction between the anhydride and the epoxy resin. Catalytic amounts of tertiary amines are commonly used as additives as they facilitate the opening of the anhydride. Anhydride epoxy formulations do not readily cure at room temperature, and generally require anelevated temperature of 80-150° C.
In the third method, the epoxy resin reacts in the ambient with polyvalent nucleophiles such as polyamines to form a polymeric network of essentially infinite molecular weight.
Polyamines of the general formula (NH2—R—NH2) give cold curing compositions. The ring opening of the epoxy ring with a primary or secondary amine generates a stable C—N bond. Epoxy groups will react with potentially every amine containing an active hydrogen atom, so that a simple diamine (NH2—R—NH2) acts as a tetrafunctional cross-linker and reacts with four epoxy groups. Similar to amines, polythiol compounds (HS—R—SH) also react with epoxy rings to form C—S bonds. The reaction of the thiol group with the epoxy group is greatly facilitated by the presence of a catalytic amount of base, such as a tertiary amine. A simple dithiol compound (HS—R—SH) serves only as difunctional chain extender since a primary thiol contains only one active hydrogen atom, but polythiol compounds with a functionality greater than three serve as cross-linkers. Polythiol hardeners also allow for ambient curing compositions. Faster setting formulations, which are commonly sold as two-pack glues in hardware stores, usually contain polythiol hardeners or both polythiol and polyamine hardeners.
By far, the most common epoxy formulations consist of a diepoxide (“resin”) and a polyamine (“hardener”) to form a polymeric network of essentially infinite molecular weight. The combination of “resin and hardener” is sometimes referred to as “cured epoxy,” “cured resin,” or simply “resin” or “epoxy.” The widespread utility of such epoxy formulations is due to their excellent processability prior to curing and their excellent post-cure adhesion, mechanical strength, thermal profile, electronic properties, chemical resistance, etc. Furthermore, the high-density, infusible three-dimensional network of epoxies makes it an extremely robust material, resulting in it being the material of choice for many long-term applications. For instance, epoxy resin, due to its excellent physical and mechanical properties, electrical insulation, and adhesive performance, is widely used in composite materials, casting parts, electronics, coating, etc. At the same time, this durability makes its removal, recycling and reworkability notoriously difficult, raising concerns about the longevity of epoxy-based materials in the environment. The cross-linking reactions that occur with two convertibly used component epoxies are essentially irreversible. Therefore, the material cannot be melted and reshaped without decomposition of the material. The ordinary consumer is also aware of the intractability of epoxy adhesives and coatings; internet message boards are replete with postings and complaints on how to remove epoxy that has spilled on unwanted places or mistakenly bonded items together. Thus, there exists a need for new epoxy formulations that retain the remarkable physical properties of classical epoxies, but can be disassembled in a controlled and mild manner when desired, without damaging the underlying structure.
As epoxy adhesives are used for the assembly of a variety of common items and epoxies serve as the matrix materials for a variety of structural materials and composites, the development of such a “reworkable” material would have implications in recycling, recovery, and waste disposal. Furthermore, an easily removable epoxy could expand the use of epoxies to new consumer markets. For example, joints could be bonded with epoxy glue and any spill-over could be easily removed, even post-curing, while the joint remains bonded. As another example, epoxy based paints and varnishes could be more easily removed.
The intractability of a cured resin stems, in part, from its highly cross-linked network. If the links in the three-dimensional network can be cleaved under controlled conditions, the network can be disassembled into smaller, soluble molecules and/or polymer, therefore removing the cured resin stem. In principal, this can be accomplished through use of either a dissolvable resin or a curing agent that contains a bond capable of cleavage under a specific set of conditions. In the limited amount of prior art on this topic, the majority has focused on cleavable groups in the resin component. Epoxy formulations that possess cleavable linkages in the hardener, are particularly attractive, as those skilled in the art realize that a great deal of more flexibility exists with regard to the constituents in a hardener component, due to the resin components in most epoxies are based on bisphenol digylcidly ether (BPADGE).
Epoxy prepreg is a compound system composed of epoxy resin, curing system and the reinforcing fiber, the resin system was an uncured state as an intermediate substrate for preparing the composite. Fiber reinforced epoxy resin composite materials, especially carbon fiber composite material prepared by the epoxy prepreg has high specific strength and specific modulus, devisable performance and diversity of forming technology, which is widely used in construction materials, aerospace and civilian entertainment. By 2015, global composites production capacity will significantly increase, and exceed 10 million tons. However, how to deal with and recycle the waste of fiber composites have become a worldwide problem and thus prevented the fiber composite industry's growth, thereby constraining the sustainable development of fiber composites.
The recovery process of fiber composites have been reported roughly in the following ways: 1. High temperature thermal degradation (Thermochimica Acta 2007(454):109-115), which can recycle composite material to obtain clean filler and fiber, but requires high temperature processing and high standard equipment; 2. Fluidized bed (Applied surface science 2008(254): 2588-2593), which requires high temperature processing to obtain the clean fiber; 3. Supercutical fluid (water (Materials and design 2010(31):999-1002), alcohol (Ind. eng. chem. res. 2010(49):4535-4541) or carbon dioxide (CN102181071), for degrading epoxy resin system, which is still in the laboratory stage and far from practical industrialization; 4. Use of nitric acid (Journal of applied polymer science, 2004 (95): 1912-1916) to degrade the epoxy resin and obtain fiber with clean surface, which has strong corrosion resistance of acid like nitric acid, requires high standard equipment, and results in low operating security, high recycle cost, and difficult post-processing. In general, these methods have their limitations in varying degrees, existing disadvantages of fiber shortening, performance degradation, environmental pollution, and high recycling cost and so on, therefore, effective and feasible method for the recycling of waste composite materials is still an issue to be addressed in composites field.